Understanding Low Outgassing Adhesives€¦ · adhesives. Recently, Master Bond introduced a UV-curable adhesive, one of the first in the industry to pass the standard ASTM E595 test

1Master Bond Inc. | TEL: +1 (201) 343-8983 | www.masterbond.com | [email protected]

Understanding Low Outgassing AdhesivesApplications that need to meet stringent outgassing requirements now have more adhesive options than ever.

T E C H S P O T L I G H T

One of the most common issues addressed by our technical

service department nowadays involves the outgassing

characteristics of adhesives and related encapsulation and

sealing compounds. And that’s understandable because a

growing number of high-tech industries have discovered

that their products and manufacturing processes can be

sensitive to outgassed chemical compounds. As a result,

there has been a dramatic upswing in the use of adhesives,

encapsulants and sealants that have been specially

formulated for low outgassing performance.

While it’s true that some standard adhesives do have

good outgassing performance “right out of the tube,”

today’s most stringent outgassing requirements are best

met by products formulated to pass an industry-standard

outgassing test known as ASTM E595-07.

The test’s full title is “A Standard Test Method for Total

Mass Loss and Collected Volatile Condensable Materials

from Outgassing in a Vacuum Environment” (see sidebar).

And engineers working on projects for NASA already know

all about it. For years, they have been specifying ASTM

E595-compliant adhesives and other materials for use in

space systems.

Now that a growing number of engineers with earthbound

applications are worrying about outgassing too, here’s a

look at what to expect from low outgassing adhesives,

encapsulants and sealants:

1

PROPERTIES

APPLICATIONS

USES

CHEMISTRY

LOW OUTGASSING

ADHESIVES

Epoxies UV Curables

OpticalAerospace Electronics

PottingBonding Sealing

Medical

ElectricalStrength

• High Lap Shear Strength

• Outstanding Peel Strength

• Superb Compressive Strength

• Electrically Insulative

• Electrically Conductive

• Thermal Conductive/ Electrically Insulative

• High Temperature Resistance

• Cryogenic Serviceability

• Withstands Thermal Cycling

• Resistant To Numerous Acids

• Resistant to Fuels and Oils

• Withstands Many Organic And Inorganic Solvents

• Excellent Optical Transmission

• Wide Range of Refractive Indices

• Superior Longevity

Thermal Chemical Optical

Figure 1. Adhesives that have passed

the industry-standard

outgassing test (ASTM

E595) offer a range of

physical properties tailored

to aerospace, optical and

electronic applications.


The Right Applications

The first thing to understand about low outgassing

products is when to use them (see “Typical Outgassing

Applications” for a full list). Other than space systems,

the most common applications for low-outgassing

products are those that involve optical or electro-

optical components, which can be clouded or fogged

by outgassed chemical compounds. Outgassing can also

affect some sensitive electrical processes—by interfering

with electrical continuity. Semiconductor manufacturing

processes can be very sensitive to outgassing material

systems. And commercial vacuum processes can also suffer

from outgassing issues.

If you suspect outgassing could potentially pose a problem

in your application, specifying adhesives that comply with

the ASTM outgassing standard will make sense as long

as one of the compliant grades meets all of your other

technical requirements. And the good news here is that low

outgassing adhesives don’t typically require any sacrifice

in other mechanical or physical properties. What sets low

outgassing products apart from their generic counterparts

is that the low outgassing grades have passed the rigorous

ASTM test—often after months of painstaking formulation

work on our part.

The time and expense of all that testing and custom

formulation do not have major cost implications for users

of low outgassing products. The compelling reason to

carefully consider whether you truly need a ASTM E595-

compliant grade comes down to freedom of choice, not cost.

Consider that Master Bond has hundreds of adhesives

that have not been tested for low outgassing and over 20

adhesives that have. By picking an ASTM E595-compliant

grade when you do not need one, you may limit your

ability to find the best match between your application

requirements and adhesive properties. For this reason,

engineers should stick with standard products for bonding,

sealing, encapsulation and potting applications that have

no known sensitivity to outgassing.

MeAsuRing OuTgAssingThe industry standard test for measuring outgassing in

adhesives and other materials is ASTM E595. Developed

by NASA to screen low-outgassing materials for use

in space, the test determines the volatile content of

material samples placed in a heated vacuum chamber.

Samples to be tested are first preconditioned at 50

percent relative humidity for 24 hours and weighed.

They then go into the test chamber for another 24

hours with the temperature set at 125 C and the

vacuum at a minimum of 5 x 10-5 torr. During that

time in the test chamber, volatiles that outgas from

the sample escape through a port in the test chamber

and condense on a cooled (25 C) collector plate. The

sample and condensate on the collector plate are

then weighed to determine the total mass lost (TML)

by the sample and the amount of collected volatile

condensable materials (CVCM) on the collector plate.

Materials pass or fail the test based on these TML and

CVCM measurements. If the CVCM exceeds 0.1 percent,

the material fails. The material will also fail if the TML

exceeds 1 percent—though the TML may be offset

by water vapor regained (WVR) by the sample in a

subsequent measurement:

• If CVCM <0.1 percent and TML < 1 percent, the

material passes.

• If CVCM <0.1 percent and TML > 1 percent, the

material can pass if the TML-WVR <1 percent.

• If CVCM > 0.1 percent or TML-WVR > 1 percent,

the material fails.

While you will occasionally run across proprietary

outgassing test methods, their results should not be

considered credible. ASTM E595 currently represents

the only widely accepted industry standard for

quantifying the outgassing performance of adhesives.

The test does have its limitations. Among them, this

24-hour test doesn’t predict long-term outgassing

performance. Nor can it determine what specific

outgassing contamination, if any, will occur in real world

operating conditions. Despite these limitations, though,

the test is a good way to evaluate and compare the

outgassing potential of adhesives and other materials.

Right: Master Bond has developed UV-curable adhesives

that pass ASTM E595, something these fast-curing

adhesives have traditionally not been able to accomplish.

This development brings the assembly speed of UV-curable

adhesives to outgassing-sensitive applications.


With that caveat in mind, the compliant adhesive products

available today do cover a wide range of capabilities for

those times when nothing else will do (see Figure 1 and

Table 1). Master Bond offers ASTM E595 products for

structural bonding, potting, encapsulation and sealing.

Also available are E595-certified electrically and thermally

conductive products as well as medical and cryogenic

adhesives. Recently, Master Bond introduced a UV-curable

adhesive, one of the first in the industry to pass the

standard ASTM E595 test.

Outgassing Tendencies

When looking for low-outgassing products, keep in mind

that not all adhesives start on a level playing field when it

comes to passing ASTM E595. Some adhesives, such as

two-part epoxies, can be specially formulated to achieve

outgassing levels well below what ASTM E595 requires.

Other types of adhesive chemistries have traditionally

not been able to pass ASTM 3595. UV-curable adhesives

once fell into this category, which kept these fast-curing

adhesives out of some otherwise suitable applications. Only

recently have Master Bond’s chemists been able to “crack

the code” of UV-curables that do pass the test.

The adhesives that outgas most severely are those that

cure through the action of solvents or moisture. These

would include a variety of pressure-sensitive and contact

adhesives as well as cyanoacrylates. These types of

adhesives are not be the best choice in applications that

have a known sensitivity to outgassing.

It’s worth noting that even within a given family of adhesives,

the outgassing potential of the individual adhesive grades

can vary substantially. This variance results from the

optimization of each grade’s physical and mechanical

properties through modifications to the underlying

adhesive chemistry. To take one common example, the

more flexible grades within the epoxy family tend to be

more prone to outgassing—possibly because they have a

lower crosslink density than their more rigid counterparts.

At the end of the day, adhesives are best looked at as

individuals rather than families when evaluating outgassing

performance. And the only way to know for certain whether

a specific adhesive meets the low outgassing criteria

spelled out in ASTM E595 is to test it.

Cure For Outgassing

When selecting adhesives for low-outgassing applications,

engineers often don’t pay much attention to cure schedules.

That’s understandable since so many modern adhesive

formulations do indeed cure well at room temperature,

developing better than adequate mechanical and physical

properties for their intended use. Yet the addition of a heat

cycle will optimize a host of desirable physical properties—

low outgassing among them. With adhesives, in other

words, it’s often the case that a room temperature cure is

good, but a heat cure is better.

In low-outgassing applications, heat can even be essential.

Heat tends to improve the crosslink density, which in

turn limits outgassing. We’ve seen cases where individual

adhesives did not pass ASTM E595 when cured at room

temperature but passed after a heat cycle. For that reason,

we recommend the addition of a heat cycle for applications

where outgassing performance is critical. A typical cure

schedule would be room temperature overnight, followed

by 2–5 hours at 60-80 C.

More information

For a guide to Master Bond’s line of ASTM-E595 compliant

epoxy adhesives, visit http://www.masterbond.com/sg/

masterbond_losg.pdf

For more information on Master Bond’s new UV-curable

ASTM-E595-compliant adhesive, click here.

For further information on this article, for answers to any adhesives applications questions, or for information on any Master Bond products, please contact our technical experts at Tel: (201) 343-8983

• Disk drive assemblies • Voice coil motors • MEMs devices • High-vacuum

applications • Solar arrays and panels • Fiber optic assemblies• LEDs • Optical telescopes • Reflectors • Respirators • Space systems • Spectrometers• Optics used in aircraft

and space applications • Lens mounts and prisms • Wavelength-division

multiplexing (WDM) applications

• Avalanche photodiodes (APDs)

• Medical filter applications

• High-power voltage supplies

• Optoelectronic assemblies

• Piezo devices• Clean room applications • Satellite assemblies • Electronics underfill

applications • Laser applications • Mirrors (including

mirrors for telescopes) • Bonding voice coil to

suspension arm • Broadband and high-

frequency RF power applications

TypiCAl lOw-OuTgAssing AppliCATiOns


TAble 1. pOpulAR lOw OuTgAssing AdhesivesMaster Bond offers a wide variety of certified low-outgassing adhesives.

Some of the most popular grades are described here:

ep29lpspLow viscosity, cryogenically serviceable epoxy with excellent physical properties

for bonding or potting. Withstands cryogenic shocks. Superior electrical insulation

properties. Optically clear.

ep21TChT-1Thermally conductive, two-component epoxy with paste consistency. Cryogenically

serviceable. High temperature resistant for bonding and sealing. Cures at room

temperature. Service temperature range 4K to 400°F.

ep37-3FlFAOThermally conductive, lower viscosity, easy to use epoxy system that cures flexible. Resistant

to vibration, impact and shock. For potting and bonding. Cryogenically serviceable.

ep30-2Superior physical properties, excellent optical transmission characteristics,

outstanding chemical resistance profile. Low viscosity enables easy application.

Service temperature range 4K to 300°F.

supReMe 10hTOne part, no-mix system requiring oven curing at 250° to 300°F with extraordinary

physical properties. High peel and tensile shear strength. Cryogenically serviceable.

Resistant to vibration, thermal shock and impact.

uv10TKlO-2One component UV curable adhesive/sealant. Exceptionally fast curing. Excellent clarity and

light transmission properties. Service temperature range -60°F to 300°F. Easy application.

ep21TdC-2AOHighly flexibilized thermally conductive two-component epoxy system. High shear and

peel strength. Superior electrical insulation and heat dissipation properties. Cures at room

temperature. Ideal for bonding dissimilar substrates. Resistant to thermal cycling.

supReMe 10hTsHigh purity silver conductive epoxy adhesive/sealant. Service temperature range from 4K to

400°F. One part, no mix system. Resists vibration, shock and thermal cycling. 100% reactive.

High peel and shear strength properties.

supReMe 10AOhT-lO

Resists up to 400°F. High thermal conductivity and excellent electrical insulation properties.

Cryogenically serviceable. One-component, no-mix system. Resists exposure to many

chemicals and thermal cycling. Cures at elevated temperatures. Low ionics.

RoHS Compliant

RoHSCOMPLIANT

Documents

Understanding Low Outgassing Adhesives€¦ · adhesives. Recently, Master Bond introduced a UV-curable adhesive, one of the first in the industry to pass the standard ASTM E595 test